A Switched-Capacitor Interface Based on Dual-Slope Integration

이중-적분을 이용한 용량형 센서용 스위치드-캐패시터 인터페이스

A novel switched-capacitor circuit for interfacing capacitive microtransducers with a digital system is developed based on the dual-slope integration. It consists of a differential integrator and a comparator. Driven by the teo phase clock, the circuit first senses the capacitance difference between the transducer and the reference capacitor in the form of charge, and accumulates it into the feedback capabitor of the integrator for a fixed period of time. The resulant accumulated charge is next extracted by the known reference charge until the integrator output voltage refurns to zero. The length of time required for the integrator output to return to zero, as measured by the number of clock cycle gated into a counter is proportional to the capacitance difference, averaged over the integration period. The whole operation is insensitive to the reference voltage and the capacitor values involved in the circuit, Thus the proposed circuit permits an accurate differental capacitance measurement. An error analysis has showh that the resolution as high as 8 bits can be expected by realizing the circuit in a monolithic MOS IC form. Besides the accuracy, it features the small device count integrable onto a small chip area. The circuit is thus particularly suitadble for the on-chip interface.